It is sometimes clinically important to determine the extent to which a patient's antibiotic-containing serum can be diluted and still kill an infecting bacterium isolated from than patient. Such a determination provides a measure of the adequacy of antibiotic treatment. No satisfactory standardized technique has been established to determine such a serum bactericidal titer, and the reproducibility of the test has been poor. By careful attention to technique and an analysis of the component technical steps, progress toward a reproducible microdilution procedure has been made. Critical factors delineated thus far include the nature of the vessel in which the test is performed, the need to introduce the inoculum for the test below the meniscus of antibiotic- containing serum and growth medium, the presence of serum in the medium. A very reproducible aqueous system has been developed, but the addition of either human or horse serum has altered the results considerably. The problems associated with addition of serum are being studied in hopes of making this a reproducible test, the clinical usefulness of which can ten be studied.